1. Field of the Invention
This invention relates to attitude control for zero momentum spacecraft and more particularly to minimizing attitude errors during periods of reaction wheel speed reversals by the addition of compensation terms to the appropriate wheel torque command.
2. Description of the Prior Art
Precise pointing earth oriented satellites may use an orthogonal reaction wheel set for attitude control. Precise three-axis attitude control is achieved by means for control torques developed by the three reaction wheels in response to sensor-derived attitude error information. The three reaction wheels are usually mounted in the spacecraft in such a manner that their axes are parallel to an orthogonal set of body axes about which attitude control is to be maintained. For an earth-pointing spacecraft, one of the wheel axes is parallel to the spacecraft pitch axis which nominally rotates about the orbit normal at one revolution per orbit. The other two wheel axes are aligned with the spacecraft roll and yaw axes, respectively. In the absence of large external disturbance torques, such a spacecraft system is referred to as zero-momentum system because the reaction wheel momentum variations about zero would be minimal when the spacecraft products of inertia are small. The roll and yaw axis reaction wheels interchange their stored angular momenta on a quarter orbit basis. This means that each of the two wheels reverse its speed direction twice per orbit. Speed direction reversals introduce excessive attitude disturbances because of internal wheel static and coulomb friction and cogging torque. The wheel may come to a complete stop for extended periods of time if the available motor torque is insufficient to overcome the effect of the internal wheel friction and torque. Many systems have been proposed heretofore for avoiding the problems associated with zero wheel speed. Some prior art attitude control systems use at least two reaction wheels on each axis to prevent wheel speed reversals. In the absence of external disturbance and internal gyroscopic cross-coupling torques, each wheel rotates in the opposite direction to its respective twin, with both wheels running at the same preselected speed. Thus, in the unperturbed case, the net stored momentum for each pair of wheels is zero. In the presence of external disturbance and internal gyroscopic cross-coupling torques, one of the two wheels provides the necessary control torque by changing its wheel speed magnitude so that the net difference in wheel speed of each wheel pair represents the total angular momentum stored along the respective spacecraft axis. By proper selection of wheel speed magnitude and momentum unloading logic, wheel speed direction reversal can be avoided. Other prior art attitude control systems use combinations of control moment gyroscopes on each axis to obviate the need for wheel speed reversals. Each of the enumerated prior art techniques have basic disadvantages in that both multiple reaction wheels and control moment gyroscopes entail additional weight, power, cost, and size. In addition control moment gyroscopes require more complex control laws. Accordingly, a system for compensating for errors introduced by wheel speed reversals is needed to overcome the deficiencies of the presently known systems.